One hundred and fifty years ago tomorrow, a small group of scientists at a meeting on the north side of Piccadilly heard the first public account of the theory that the philosopher Daniel Dennett calls the single best idea that anyone has ever had.

The reading was precipitated by an event many thousands of miles away on the island of Ternate (now in Indonesia). The young explorer Alfred Russel Wallace had spent a decade travelling the globe amassing a collection of more than 100,000 specimens of plants and animals. He realised the astonishing variety that filled his collection boxes could all be accounted for by the simple fact that fitter individuals leave more descendants. Wallace scribbled off an account of his idea to the greatest naturalist of the day, Charles Darwin, and this note triggered the unveiling of Darwin's theory of natural selection in London. Darwin had been working on the theory for 20 years but had published nothing. The great and good of scientific London were consulted, and it was agreed that both men's ideas should be read as letters at the Linnean Society meeting.

In truth, the joint papers did not excite much interest. Professor Haughton of Dublin commented that "all that was new in them was false, and what was true was old". It is only after Darwin's The Origin of Species was published the following year that the significance of the theory of natural selection sank in.

Quite simply, Darwin and Wallace destroyed the strongest evidence left in the 19th century for the existence of a deity. Two centuries earlier, Newton had banished God from the clockwork heavens. Darwin and Wallace made the deity equally redundant on the surface of the earth. Ever since, biologists have used Darwin's theory to make sense of the natural world. Contrary to the arguments of creationists (and "intelligent design" advocates) evolution is no longer just a theory. It is as much a fact as gravity or erosion. Scientists have measured evolutionary changes in scores of organisms.

Of course, puzzles still remain. The origin of life itself remains a deep mystery. Although natural selection can (over many millions of years) transform a self-replicating microbe into a mongoose, a manta ray or a man, it cannot, on its own, generate a self-replicator.

But perhaps the most hotly debated issue in evolutionary biology lies the other end of the biological timescale, with the emergence of the human brain and its extraordinary capacity for abstract thought. It is easy to see a selective advantage for intelligence or the skill to make tools but what about a capacity to appreciate art, weep at a sad song or perform long division? What use would these skills have been to our distant ancestors?

With The Selfish Gene, published in 1976, Richard Dawkins argued that many cherished aspects of our humanity, such as our capacity for kindness, are programmed into our genes. This genocentric view was strenuously resisted by Stephen Jay Gould, who stressed the independence of the individual and his or her mind over their inherited genes.

Perhaps curiously, the latest buzz- word in biology - systems biology- may weigh in on the side of the individual. The key insight of systems biology is that genes don't really exist in isolation but only as components of complex systems of cells and organisms. In this sense genes, selfish or not, are unlikely units of natural selection. So the individual, rather than his or her genes, may finally have the last word. As naturalists who spent their lives studying biological form, Darwin and Wallace would surely have been delighted with this renewed interest in whole organisms, rather than their parts.